Technical Field
Embodiments of the subject matter disclosed herein generally relate to attenuating vibrations in marine seismic acquisition equipment. More specifically, the embodiments relate to noise cancellation by actively attenuating axial and/or transverse vibrations in marine seismic acquisition equipment.
Discussion of the Background
Seismic data acquisition and processing generate a profile (image) of geophysical structures under seafloor or subsoil by emitting waves towards the seafloor and receiving and processing the reflections. FIG. 1 is a schematic diagram of a seismic survey system 100 used for seismic data acquisition. The system includes a survey vessel 102, which tows a source array 118 with source elements that generate waves directed at the surveyed area and a plurality of streamers 104, each of the streamers having one or more receivers 106 for receiving reflections from the surveyed area. The source elements can be vibrators, sparkers, explosives, or an electromagnetic source. The receivers 106 can be hydrophones, geophones, accelerometers, or electromagnetic sensors.
The streamers 104 are coupled to the survey vessel 102 by front-end gear, which includes a plurality of lead-in cables 108. Spread ropes 110 laterally couple adjacent lead-in cables 108 to each other and are designed to prevent the distance between adjacent streamers from exceeding a desired distance. The lead-in cables 108 extend beyond the spread rope 110 and include a float 112, coupled to a bend restrictor 114, to lift the streamers 104 to an intended depth. Wide-tow ropes 120 are connected to spurline 122 outside of the lead-ins 108. Unlike lead-ins 108, wide-tow ropes 120 do not carry an electrical connection. Wings 116 are connected to the spurline 122 via a lever-arm (not illustrated) to achieve parallel trajectories for the streamers 104 in the towing direction and also to achieve a desired separation between the center-most lead-in cables.
FIG. 2 is a side view of a streamer when deployed under water. Buoys 112, 142 maintain the streamers 104 substantially parallel relative to a reference plane (e.g., the water surface). Specifically, a head-buoy 112 is connected to a head portion 104A of the streamer 104 and a tail-float 142 is connected at a tail portion 104B of the streamer 104. The head-buoy and tail-float provide flotation to the streamer even if the streamer is buoyant neutral. The head-buoy and tail-float are configured to float at the water surface 150 and corresponding cable 140 (for mechanical purposes) connects the head-buoy 112 to the streamer 104 to maintain the streamer at the desired depth H.
The head-buoy 112 is equipped with various equipment, e.g., acoustic equipment for detecting positions of neighboring streamers and global positioning system (GPS) equipment for determining an absolute position of the streamer. In order to power the equipment, electric power generated on the towing vessel may be transferred through an electric cable 152 to the head-buoy 112. The electric cable 152 and the cable 140 connect to the streamer 104 through a connection device 154.
As the streamers are towed through the water the front-end typically generates vibrations that are transferred to the streamers. The vibrations can be generated both axially, i.e., along the length of the streamer, and transversely, i.e., in the direction between the streamer and the water surface. These vibrations generate radial noise, which impacts the signals received by the streamers.
One conventional solution to address vibrations generated by the front-end is to incorporate elastic sections, commonly referred to as Vibration Isolation Modules (VIMs), between the lead-in cables and the streamers. For example, in FIGS. 1 and 2 VIMs 124 are arranged between the lead-ins 108 and streamers 104. These elastic sections, however, have a constant elasticity and can be optimized to dampen only a limited range of frequencies. Further, these elastic sections generally only dampen axial vibrations and have little dampening effect in the transverse direction.
Accordingly, it would be desirable to provide devices, systems and methods to attenuate vibrations generated by the front-end while avoiding the afore-described problems and drawbacks.